1. Field of the Invention
The present invention relates to an optical pulse time spreader which is used as an encoder or decoder in an optical code division multiplexing (OCDM) communication system, and more particularly, to an optical pulse time spreader where unit diffraction gratings are arrayed in series along the wave guiding direction of an optical wave guide.
2. Description of Related Art
Today demands for communication is rapidly increasing due to the spread of the Internet and other reasons. To support this, high-speed and large capacity networks using optical fibers are being provided. And to increase the capacity of communication, optical multiplexing technology for transmitting a plurality of channels of optical pulse signals simultaneously using one optical fiber transmission line is under consideration.
As one optical multiplexing technology, OCDM is under research. A feature of OCDM is flexibility in operation, that is, no restrictions in the number of optical pulse signals assigned per bit on the time base. Another feature is that a plurality of channels can be set for a same time slot on a time base, or that a plurality of communication channels can be set for a same wavelength on a wavelength base.
OCDM is a communication method for assigning a different code (pattern) to each channel and extracting signals by pattern matching. In other words, OCDM is an optical multiplexing technology for encoding the optical pulse signal by an optical code which is different for each communication channel at the transmission side, and for decoding the encoded optical pulse signal using a same optical code as the transmission side so as to return it to the original optical pulse signal at the receive side.
Since only the optical pulse signal of which code matches is extracted and processed as an effective signal during decoding, the optical pulse signal comprised of lights with a same wavelength, or with a plurality of wavelengths combined, can be assigned to a plurality of communication channels. For the encoder, a passive optical element, such as a Fiber-Bragg-Grating (FBG) can be used for the phase control required for code processing, so the communication rate can be faster without receiving electric limitation in code processing. Also a plurality of channels can be multiplexed with a same wavelength at a same point in time, and large capacity data can be communicated.
As a means of encoding in OCDM, an optical phase encoding method, which uses a phase of light as a code, is known. Specifically, a superstructure Fiber-Bragg-Grating (SSFBG) is used for the encoder and decoder (e.g. see Toru Mizunami: “Optical fiber diffraction grating”, Applied Physics, Vol. 67, No. 9, pp. 1029-1034 (1998), or Akihiko Nishiki, Hideyuki Iwamura, Shuko Kobayashi, Satoko Kutsuzawa, and Saeko Oshiba: “Development of Encodr/Decoder for OCDM using a SSFBG”, Technical Report of IEICE, OFT 2002-66, (2002-11), or Hideyuki Sotobayashi: “Optical code division multiplexing network”, Applied Physics, Vol. 71, No. 7, pp. 853-859 (2002)).
Now the operation principle of the case when the optical pulse time spreader, which uses SSFBG as the phase control means, is used as an encoder and decoder will be described with reference to FIGS. 1A to 1E. Hereafter the optical pulse time spreader which uses SSFBG as a phase control means may simply be called a “SSFBG optical pulse time spreader”. FIG. 1A is a diagram depicting the time-based waveform of an input optical pulse. FIG. 1E is a diagram for explaining the state when an encoded optical pulse string encoded by an encoder is decoded by a decoder.
The input optical pulse shown in FIG. 1A is input to an encoder 10 from an optical fiber 12 via an optical circulator 14 as shown in FIG. 1E, and is encoded there, then propagates an optical fiber 18 via the optical circulator 14 again, and is input to a decoder 20 via an optical circulator 22. And auto-correlation waveforms are generated by the optical pulse being decoded by the decoder 20, and this auto-correlation waveform propagates an optical fiber 26 via the optical circulator 22.
The encoder 10 and decoder 20 shown in FIG. 1E are SSFBGs comprised of 4 unit FBGs being arrayed along the wave guiding direction of the optical fiber. Here the functions of the encoder 10 and decoder 20 will be described using 4-bit optical code (0, 0, 1, 0) as an example. The number of terms of the sequence formed by “0s” and “1s” which provide the optical code may be called a “code length”. In this example the code length is 4. The sequence to provide an optical code may be called a “code string”, and each term “0” and “1” of the code string may be called a “chip”. And the value of 0 or 1 itself may be called a “code value”.
The unit FBG 10a, 10b, 10c and 10d, constituting the encoder 10, corresponds to the first chip “0”, second chip “0”, third chip “1” and fourth chip “0” of the optical code respectively. Whether the code value is 0 or 1 is determined by the phase relationship of the Bragg-reflected light, which is reflected from the adjacent unit FBG. In other words, the first chip and second chip have the same code value 0, so the phase of the Bragg-reflected light reflected from the unit FBG 10a which corresponds to the first chip, and the phase of the Bragg-reflected light reflected from the unit FBG 10b which corresponds to the second chip are the same. The code value of the second chip is 0 and the code value of the third chip is 1, so these chips have different values. Therefore the difference between the phase of the Bragg-reflected light reflected from the unit FBG 10b which corresponds to the second chip and the phase of the Brigg-reflected light reflected from the unit FBG 10c which corresponds to the third chip is π. In the same way, the code value of the third chip is 1 and the code value of the fourth chip is 0, so these chips have different values. Therefore the difference between the phase of the Bragg-reflected light reflected from the unit FBG 10c which corresponds to the third chip and the phase of the Brigg reflected light reflected from the unit FBG 10d which corresponds to the fourth chip is π.
This optical code defined by changing the phase of the Bragg-reflected light from the unit FBG may be called an “optical phase code”.
Now the steps of the formation of the auto-correlation waveform by the optical pulses being encoded by the encoder and converted into an encoded optical pulse string and the encoded optical pulse string being decoded by the decoder will be described. When a single optical pulse shown in FIG. 1A is input from the optical fiber 12 to the encoder 10 via the optical circulator 14 and the optical fiber 16, the Bragg-reflected lights from the unit FBGs 10a, 10b, 10c and 10d are generated. These Bragg-reflected lights from the unit FBGs 10a, 10b, 10c and 10d are assumed to be a, b, c and d respectively. In other words, the single optical pulse shown in FIG. 1A is time-spread to Bragg-reflected lights a, b, c and d, and converted into an encoded optical pulse string.
The Bragg-reflected lights a, b, c and d are separated into 4 optical pulses, and constitute an optical pulse string where the optical pulses are arrayed with a specific interval which depends on the way of arraying the unit FBGs 10a, 10b, 10c and 10d on the time base, as shown in FIG. 1B. Therefore the encoded optical pulse string is an optical pulse string, which is the optical pulse input to the encoder time-spread into a plurality of optical pulses on the time base.
FIG. 1B shows the encoded optical pulse string propagating the optical fiber 18 with respect to the time base. In FIG. 1B, the optical pulses are shifted in the ordinate direction so as to clearly show the encoded optical pulse string.
The Bragg-reflected light by the unit FBG 10a is the optical pulse indicated by a in FIG. 1B. In the same way, the Bragg-reflected lights by FBG 10b, FGB 10c and FGB 10d are optical pulses indicated by b, c and d respectively in FIG. 1B. The optical pulse indicated by a is an optical pulse reflected from the unit FBG 10a, which is closest to the input end of the encoder 10, so this pulse is at a position most advanced on the time base. The optical pulses indicated by b, c and d are Bragg-reflected lights from FBG 10b, FBG 10c and FBG 10d, and FBG 10b, FBG 10c and FBG 10d are arrayed in this sequence from the input end of the encoder 10, so the optical pulses indicated by b, c and d are arrayed in the sequence of b, c and d after the optical pulse indicated by a, as shown in FIG. 1B. In the following description, the optical pulses corresponding to the Bragg-reflected light a, Bragg-reflected light b, Bragg-reflected light c and Bragg-reflected light d may be called the “optical pulse a, optical pulse b, optical pulse c and optical pulse d” respectively. The optical pulse a, optical pulse b, optical pulse c and optical pulse d may be called the “chip pulses”.
The phase relationship of the Bragg-reflected lights a, b, c and d, constituting the encoded optical pulse string which was described above, are as follows. The phases of the Bragg-reflected light a and the phase of the Bragg-reflected light b are the same. The difference of the phase of the Bragg-reflected light b and the phase of the Bragg-reflected light c is π. The difference of the phase of the Bragg-reflected light c and the phase of the Bragg-reflected light d is π. In other words, if the phase of the Bragg-reflected light a is used as a reference, the phases of the Bragg-reflected light a, Bragg-reflected light b and Bragg-reflected light d are the same, and the phase of the Bragg-reflected light c is different from those by π.
In FIG. 1B, the optical pulses corresponding to the Bragg-reflected light a, Bragg-reflected light b and Bragg-reflected light d are shown by a solid line, and the optical pulse corresponding to the Bragg-reflected light c is shown by a broken line. In other words, the sold line and the broken line are used to indicate the corresponding optical pulses in order to distinguish the phase relationships of the Bragg-reflected lights. The phases of the optical pulses indicated by the sold line are the same, and the phases of the optical pulses indicated by the broken line are the same. And the phase of the optical pulses indicated by the solid line and the phase of the optical pulse indicated by the broken line are different from each other by π.
The encoded optical pulse string propagates the optical fiber 18 and is input to the decoder 20 via the optical circulator 22. The decoder 20 has the same structure as the encoder 10, but the input end and output end are reversed. In other words, the unit FBGs 20a, 20b, 20c and 20d are arrayed sequentially from the input end in the decoder 20, and the unit FBG 20a corresponds to the unit FBG 10d. In the same way, the unit FBG 20b, unit FBG 20c and unit FBG 20d correspond to the unit FBG 10c, unit FBG 10b and unit FBG 10a respectively.
For the encoded optical pulse string to be input to the decoder 20, the optical pulse a constituting this encoded optical pulse string are first Bragg-reflected from the unit FBGs 20a, 20b, 20c and 20d respectively. This state will be described with reference to FIG. 1C. In FIG. 1C, the abscissa is the time base. And 1 to 7 are assigned for convenience to indicate the sequence in time.
FIG. 1C is a diagram depicting the encoded optical pulse string with respect to the time base, just like FIG. 1B. The encoded optical pulse string which is input to the decoder 20 is Bragg-reflected by the unit FBG 20a. The reflected light which was Bragg-reflected by the unit FBG 20a is called “Bragg-reflected light a′”. In the same way, the reflected lights, which were Bragg-reflected by the unit FBG 20b, unit FBG 20c and unit FBG 20d are called “Bragg-reflected lights b′, c′ and d′” respectively.
From the unit FBG 20a, the optical pulses a, b, c and d, constituting the encoded optical pulse string, are Bragg-reflected and arrayed in the string a′ on the time base in FIG. 1C. The optical pulse a, Bragg-reflected by the unit FBG 20a, is an optical pulse of which peak is at the position indicated by 1 on the time base. The optical pulse b, Bragg-reflected by the unit FBG 20a, is an optical pulse of which peak is at the position indicated by 2 on the time base. And in the same way, the optical pulse c and optical pulse d are optical pulses of which peaks are at the positions indicated by 3 and 4 on the time base respectively.
From the unit FBG 20b as well, the optical pulses a, b, c and d constituting the encoded optical pulse string are Bragg-reflected, and arrayed in the string b′ on the time base in FIG. 1C. Compared with the Bragg-reflected lights a′, c′ and d′, the phase of the Bragg-reflected light b′, which is reflected from the unit FBG 20b, is shifted by π. Therefore the phases of the optical pulse string arrayed on the string a′ on the time base and the phases of the optical pulse string arrayed on the string b′ on the time base are all shifted by π.
Therefore the optical pulses arrayed in the sequence of 1 to 4 in the string a′ on the time base are arrayed in the sequence of solid line, solid line, broken line and solid line, but the optical pulses arrayed in the sequence of 2 to 5 in the string b′ on the time base are arrayed in the sequence of broken line, broken line, solid line and broken line. The optical pulse string a′ and optical pulse string b′ are shifted on the time base, because among the optical pulses constituting an encoded optical pulse string, the optical pulse a is input to the decoder 20 before the optical pulse b.
In the same way, from the unit FBG 20c and unit FBG 20d, the optical pulses a, b, c and d constituting the encoded optical pulse string are Bragg-reflected, and arrayed in the strings c′ and d′ on the time base in FIG. 1C respectively. The phases of the Bragg-reflected lights c′ and d′ which are reflected from the unit FBG 20c and the unit FBG 20d are the same as the Bragg-reflected light a′. Therefore the optical pulse string c′ and the optical pulse string d′ are arrayed on the time base in FIG. 1C. The optical pulses related to the Bragg-reflected lights a′, c′ and d′ are shifted in parallel on the time base, but the mutual phase relationship of the respective optical pulses related to the Bragg-reflected lights are the same.
FIG. 1D shows an auto-correlation waveform of the input optical pulse decoded by the decoder 20. The abscissa is a time base, which corresponds to the diagram in FIG. 1C. The auto-correlation waveform is given by the sum of the Bragg-reflected lights a′, b′, c′ and d′ from each unit FBG of the encoder, so it is all the Bragg-reflected lights a′, b′, c′ and d′ shown in FIG. 1C combined. At the time indicated by 4 on the time base in FIG. 1C, all the optical pulses related to the Bragg-reflected lights a′, b′, c′ and d′ are added at the same phase, so the maximum peak is formed. At the time indicated by 3 and 5 on the time base in FIG. 1C, two optical pulses indicated by the broken line and one optical pulse indicated by the solid line area added, so a peak the same as one optical pulse, of which phase is different from the maximum peak, which is located at the time indicated by 4, by π is formed. At the time indicated by 1 and 7 on the time base in FIG. 1C, there is one optical pulse indicated by the solid line, so a peak the same as one optical pulse of which phase is the same as the maximum peak, which is located at the time indicated by 4, is formed.
As described above, the optical pulse is encoded by then encoder 10, and becomes the encoded optical pulse string, and this encoded optical pulse string is decoded by the decoder 20, and an auto-correlation waveform is generated. In the above example, 4-bits (code lengths 4) of optical code (0, 0, 1, 0,) was used, but the above description is still valid even if the optical code is different from this.
Now the general structure of SSFBG, which is the phase control means of a conventional optical pulse time spreader, will be described with reference to FIGS. 2A to 2C. FIG. 2A is a cross-sectional view depicting an SSFBG. This SSFBG has an SSFBG 30 installed at the core 34 of an optical fiber 36, which is comprised of the core 34 and the clad 32. The SSFBG 30 is comprised of 15 unit FBGs arrayed in series along the wave guiding direction of the core 34, which is an optical wave guide of the optical fiber 36.
The optical phase code which is set in the conventional SSFBG in FIG. 2A is (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1) in 15-bit code string notation. And the correspondence of the 15 unit FBGs arrayed in series in the core 34 and the above optical codes are as follows. The unit FBGs arrayed in the direction from the left end to the right end of the SSFBG 30 in FIG. 2A and the chips arrayed in the direction from the left end to the right end of (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1), which indicate the optical codes of the unit FBG denoted as a 15-bit code string, correspond one-to-one.
The refractive index modulation period Λ of the 15 unit FBGs and the Bragg reflection wavelength λ have the relationship λ=2neff·Λ. Here neff is an effective refractive index with respect to the light guided through the core 34. In the following description, the effective refractive index may be simply called the “refractive index”. In other words, the refractive index modulation structure of SSFBG 30 in FIG. 2B, which will be described later, refers to the modulation structure of the refractive index of the SSFBG 30.
In FIG. 2A, there are 2 cases of the phase relationship of the Bragg-reflected light, which is reflected from the adjacent unit FBG, that is the case of having phase difference 0, and the case of having phase difference π. In FIG. 2A, if a number from 1 to 15 is sequentially assigned to the unit FBGs arrayed from the left to right, so as to be unit FBG 1, unit FBG 2, . . . , unit FBG 15, the phase relationship of the Bragg-reflected light, which is reflected from the adjacent unit FBG, can be set as shown in Table 1 in order to set 15-bit code string (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1).
The unit FBG for generating an adjacent chip pulse, of which phase difference is π, is an adjacent unit FBG, and the geometric arrangement interval (phase shift amount in Table 1) of these unit FBGs which are adjacent to each other is π/2 if converted into a phase value since light travels back and forth in this interval. In other words, the chip pulse (former chip pulse) generated by being Bragg-reflected by the unit FBG (former unit FBG) and the chip pulse (latter chip pulse) which is output adjacent to this chip pulse are chip pulses generated by being Brigg-reflected by the unit FBG (latter unit FBG) which is arranged next to this unit FBG (former unit FBG). Therefore the latter chip pulse is output later by the time for the pulse to travel back and forth once ((π/2)×2=π) in the interval between the former unit FBG and the latter unit FBG (π/2).
TABLE 1Code000111101011001PhaseShift Amount00  π  2000  π  2  π  2  π  2  π  20  π  20  π  2
Concerning the phase relationship of the Bragg-reflected lights which are reflected from the adjacent unit FBGs, the case when the phase difference between them is 0 and π is the case of 2πM and (2N+1)π(=2πN+π), where M and N are integers. In this case, the interval between the adjacent unit FBGs is given by πM and πN+(π/2) if they are converted into phase values. In other words, the phase difference of the Bragg-reflected light from the unit FBG, which is reflected from the adjacent unit FBG, is double the interval of these adjacent unit FBGs since light travels between these adjacent unit FBGs back and forth. However, in the following description, the phase value may be denoted simply as 0 and π/2, that is, M=N=0, omitting general notation such as πM and πN+(π/2).
In FIG. 2A, if the phases of the Bragg-reflected lights between adjacent unit FBGs are different by π, the interval between these unit FBGs is filled in black. If the phases of the Bragg-reflected lights between adjacent unit FBGs are the same, the interval between these unit FBGs is shown as a continuous optical modulation structure. In FIG. 2B, on the other hand, if the phases of the Bragg-reflected lights between adjacent unit FBGs are different by π, the interval between these unit FBGs is indicated by a black upside down triangle.
If the phases of the Bragg-reflected lights between adjacent unit FBGs are the same, the refractive index modulation structure of these unit FBGs has a continuous periodic structure. If the phases of the Bragg-reflected lights between adjacent unit FBGs are different by π, on the other hand, a shift for the amount of π (jump of π phase) is inserted at the boundary of these unit FBGs in the refractive index modulation structure of these FBGs.
Hereafter the numbers 1 to 15 assigned to each unit FBG may be called a “unit FBG number”. The unit FBG 1, unit FBG 2, . . . , unit FBG 15 may also be called the “first unit FBG, second unit FBG, . . . , fifteenth unit FBG”.
The top level of Table 1 indicates the 15-bit code string (0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1), and the unit FBG 1, unit FBG 2, . . . , unit FBG 15 correspond to these 15 code values one-to-one. The bottom level of Table 1 shows the phase relationship of the Bragg-s reflected lights reflected from the adjacent unit FBGs. For example, it is shown that the phase difference between the Bragg-reflected lights reflected from the unit FBG 1 and unit FBG 2 is 0, and in the same way, the phase difference between the Bragg-reflected lights reflected from the unit FBG 2 and unit FBG 3 is 0. Also the phase difference of the Bragg-reflected lights reflected from the unit FBG 3 and unit FBG 4 is π. This is the same for the unit FBGs other than the unit FBG 1 to unit FBG 4.
The characteristic of this SSFBG 30 is that the refractive index modulation intensity of the periodic refractive index modulation structure forming the unit FBGs which are arrayed in series along the wave guiding direction of the optical fiber is simply increased along the wave guiding direction of this optical fiber.
FIG. 2B is a diagram depicting the refractive index modulation structure of the SSFBG 30 shown in FIG. 2A. FIG. 2C is an enlarged view of a part of the refractive index modulation structure of the unit FBG shown in FIG. 2B. The level Δn of the refractive index modulation of the refractive index modulation structure of the unit FBG is simply increased along the wave guiding direction (x direction) of the optical fiber 36. In FIG. 2B, the refractive index of the unit FBG at the position where the level of the refractive index modulation is at maximum is na+(Δn/2), and the refractive index of the unit FBG at the position where the level is minimum is na−(Δn/2). Here na is an average value of the refractive index of the optical fiber 36.
In other words, in FIG. 2B, the central axis, that indicates the average refractive index na of the optical fiber, is indicated by a straight line which is parallel with the abscissa indicating the positional coordinate of the optical fiber in the length direction. In other words, the curves above this straight line indicate that the average refractive index is Δn/2 higher than na, and the curves below this straight line indicate that the average refractive index is Δn/2 lower than na. Therefore the level of the refractive index of the SSFBG 30 is expressed as na±(Δn/2) using the average refractive index na.
The relationship between the reflectance of each unit FBG and the intensity of the output chip pulse from each unit FBG will be described with reference to FIGS. 3A and 3B. In FIGS. 3A and 3B, the abscissa indicates the unit FBG number. The ordinate of FIG. 3A indicates the reflectance from each unit FBG, and the ordinate of FIG. 3B indicates the intensity of the chip pulse which is output from each unit FBG in an arbitrary scale.
Since the level Δn of the refractive index modulation simply increases along the wave guiding direction (x direction) of the optical fiber 36, the reflectance of the unit FBG increases as the unit FBG number increases, as shown in FIG. 3A. In other words, the reflectance of the unit FBG increases as the position of the FBG becomes closer to the right end of the optical fiber 36. Since the unit FBGs are arrayed like this, as shown in FIG. 3B, the intensities of the chip pulses which are output from each unit FBG can be equalized. The reason for this follows.
In other words, the optical pulse that enters the SSFBG is Bragg-reflected by the first unit FBG (unit FBG 1), and when the optical pulse enters the second unit FBG (unit FBG 2), the intensity thereof has been decreased for the amount of the intensity of the Bragg-reflected light at the first unit FBG. Therefore if the reflectance is set the same for all 15 unit FBGs, the intensity of the Bragg-reflected light at the second unit FBG becomes lower than the intensity of the Bragg-reflected light at the first unit FBG. In this way, the intensity of the Bragg-reflected light from each unit FBG sequentially decreases in the sequence of first to fifteenth unit FBG.
Therefore by simply increasing the refractive index modulation intensities of the 15 unit FBGs arrayed in series along the wave guiding direction of the optical fiber in this configuration, it is set that the Bragg reflectance of each unit FBG simply increases sequentially from the first to fifteenth unit FBG. By this, the Bragg reflectance can be increased so as to compensate the decrease of incident intensity to each unit FBG. And all the intensities of the Bragg-reflected lights from the first to fifteenth FBGs (intensities of chip pulses to be output) can be equalized.
If all the intensities of the chip pulses which are output from the first to fifteenth unit FBGs can be equalized, the time-based waveform of the encoded optical pulse string can be closer to becoming flat with respect to the time base. In other words, the encoded optical pulse is time-spread at an equal intensity by the encoder within a spreading time. If the optical pulse is time-spread to be a chip pulse string having equal intensity within the spreading time, the energy of the optical pulse can be more efficiently converted into an encoded optical pulse string compared with the case when the optical pulse is time-spread unequally. Also as mentioned later, if the SSFBG optical pulse time spreader is used as a decoder, the ratio of the peak and sub-peak of the auto-correlation waveform can be increased, and the reliability to identify the signal can be increased (e.g. see Koji Matsushima, Xu Wang, Satoko Kutsuzawa, Akihiko Nishiki, Saeko Oshiba, Naoya Wada and Ken-ichi Kitayama: “Experimental Demonstration of Performance Improvement of 127-Chip SSFBG En/Decoder Using Apodization Technique”, IEEE Photonics Technology Letters, Vol. 16, No. 9, pp. 2192-2194, September 2004). If the peak of the auto-correlation waveform is a signal, the sub-peak is a noise, so the ratio of the peak and sub-peak can be regarded as the S/N ratio (Signal-to-Noise Ratio).
However in the above mentioned conventional SSFBG, the chip pulses to be output are not always generated by a single Bragg reflection from a unit FBG (hereafter may be called a “single reflection”), but include those generated by an odd count of Bragg reflections. Therefore even if the refractive index modulation intensity of the periodic refractive index modulation structure forming the unit FBGs arrayed in series along the wave guiding direction of the optical fiber is simply increased along the wave guiding direction of the optical fiber, the optical pulses to be input are not always output as a chip pulse string time-spread at equal intensity.
This will be described with reference to FIGS. 4A to 4C FIGS. 4A to 4C are diagrams for explaining the effect of multiple reflection on the output chip pulse intensity generated in the unit FGB. The abscissa of FIGS. 4A to 4C indicates a time base in an arbitrary scale, and the ordinate, which is omitted, indicates the light intensity in an arbitrary scale.
FIGS. 4A to 4C show the time-based waveform of a chip pulse string, which is output from the SSFBG as a Bragg-reflected light when one optical pulse is input to the SSFBG. FIG. 4A shows a time-based waveform of the chip pulse string which is generated from each unit FBG by a single reflection. FIG. 4B shows a time-based waveform of the chip pulse string generated from each unit FBG by the triple Bragg reflection (hereafter may be called “triple reflection”). FIG. 4C shows a time-based waveform of a chip pulse string which is output from the SSFBG as a result of superimposing and interfering of the chip pulse string which is generated from each unit FBG by a single reflection and the chip pulse string which is output by a triple reflection.
The generation of the chip pulse string which is output from each unit FBG by triple reflection will be described.
The optical pulse which is input to the SSFBG is first Bragg-reflected by the first unit FBG (unit FBG 1), and is output. This is the chip pulse indicated by “1” in FIG. 4A. Then the optical pulse is Bragg-reflected by the second unit FBG (unit FBG 2), and is output. This is the chip pulse indicated by “2” in FIG. 4A. In the SSFBG comprised only of 2 unit FBGs, a chip pulse, which is output by triple reflection, does not exist. In FIG. 4A, the chip pulses indicated by “3” and “15” are chip pulses by single reflection, which are Bragg-reflected by unit FBG 3 to unit FBG 15 respectively, and are output. The numbers 1 to 15, which are assigned to each chip pulse as “1” to “15” in FIG. 4A, may be called “chip numbers”.
The chip pulse with chip number 3 in FIG. 4A is a chip pulse which is Bragg-reflected by the third unit FBG (unit FBG 3), and is output. The chip pulse with chip number 3 in FIG. 4B is reflected by unit FBG 2, and is reflected by unit FBG 1, and is reflected again by unit FBG 2, and is output, which is a triple reflection chip pulse. The chip pulse with chip number 4 in FIG. 4B is a result of interference of the total 3 chip pulses, that is, a triple reflection chip pulse which is reflected by unit FBG 2, is reflected by unit FBG 1, and is reflected again by unit FBG 3, then is output, a triple reflection chip pulse which is reflected by unit FBG 3, is reflected by unit FBG 1, and is reflected again by unit FBG 2, then is output, and a triple reflection chip pulse which is reflected by unit FBG 3, is reflected by unit FBG 2, and is reflected again by unit FBG 3, then is output. This is the same for chip pulses with chip numbers 5 to 15 in FIG. 4B.
In other words, the chip pulse with chip number k in FIG. 4B includes the triple reflection chip pulse which is reflected by unit FBG (k−1), is reflected by unit FBG (k−2), and is reflected again by unit FBG (k−1), then is output, and is all of the triple reflection chip pulses which are generated by three times of Bragg-reflections between a set of two unit FBGs selected from (k−1) number of unit FBGs from unit FBG 1 to unit FBG (k−1), and is output for the k-th time. Here k is an integer in the 3 to 15 range.
FIG. 4C shows the time-based waveform of the intensity of the chip pulse string, which is given by a square of the sum of the amplitude of the chip pulse shown in FIG. 4A, and the amplitude of the chip pulse shown in FIG. 4B. As FIG. 4C shows, if a chip pulse which is output by an odd count of Bragg reflections exists, the optical pulse to be input is not always output as a chip pulse string which was time-spread at equal intensity, even if the unit FBGs are arrayed so that the reflectance increases sequentially along the wave guiding direction of the optical fiber. In other words, in order to output the optical pulse as a chip pulse string which was time-spread at an equal intensity, it is not sufficient merely to array the unit FBGs so that the reflectance increases sequentially along the wave guiding direction of the optical fiber.
The result of quantitatively computing and confirming the contents of the above description is shown in FIGS. 5A and 5B. FIGS. 5A and 5B are simulation results on the intensity of a chip pulse string which is output from an SSFBG. The abscissa of FIGS. 5A and 5B indicates the chip number. The ordinate of FIG. 5A indicates the intensity of the chip pulse, and the ordinate of FIG. 5B indicates the phase of the chip pulse. The chip pulse of which phase is “−1” in the ordinate of FIG. 5B and the chip pulse of which phase is “1” are in an opposite phase relationship. In other words, the phase difference of the chip pulse of which phase is “−1” and the chip pulse of which phase is “1” is π.
In FIG. 5A, a chip pulse generated by a single reflection is indicated by a white circle, and a chip pulse generated by a triple reflection is indicated by an ×. And a chip pulse determined as a sum of the chip pulses generated by a single reflection and a triple reflection is indicated by a black dot. In FIG. 5B, a chip phase of a chip pulse generated by a single reflection is indicated by a white circle, and a phase of a chip pulse generated by a triple reflection is indicated by an ×.
As the white circles in FIG. 5A show, the intensities of the chip pulses generated by a single reflection are equalized, but as the black dots show, the intensities of chip pulses determined as the sum of the chip pulses generated by a single reflection and a triple reflection, that is chip pulses which are output from SSFBG, disperse. This is because if the phase of the chip pulse generated by a single reflection and a phase of the chip pulse which is output by a triple reflection are in an opposite phase relationship, the intensity of the chip pulse which is determined as the sum thereof and which is output from SSFBG is weakened.
As described with reference to FIGS. 1A to 1E, the decoder using SSFBG re-encodes the chip pulse string which was generated by being encoded, so as to regenerate the optical pulse from the chip pulse string which was encoded and generated by the encoder. If the intensity of the chip pulse string generated by the optical pulse being time-spread by SSFBG is not uniform, this chip pulse of which intensity is not uniform is re-encoded by the decoder using SSFBG. By this re-encoding by the decoder as well, the intensity of the generated chip pulse string is not uniform, so the optical pulse regenerated by the decoder does not have an ideal time-based waveform which has a single peak. The case when the code length is 15 was described above, but needless to say, the above description is valid even when the code length is other than 15.